The present invention relates to a direct-current motor and more particularly to its field device.
FIGS. 4A and 4B are block diagrams of a field device for a conventional direct-current motor. FIG. 4A is a side sectional view and FIG. 4B is a bottom view.
In FIGS. 4A and 4B, reference numeral 1 denotes the whole field device for a magneto direct-current motor; 2, a yoke; and 3, a field pole which is formed from a magnet secured to the inner peripheral face of the yoke 2, the field pole comprising a main magnetic pole 4 made of a ferrite permanent magnet secured on the demagnetizing side (the rear side in the direction of rotation) and an auxiliary magnetic pole 5 made of soft iron or soft ferrite secured on the magnetizing side (the front side in the direction of rotation). Further, a reference numeral 6 denotes an armature core which is located on the inner peripheral side of the field device 1 opposite to the field pole 3 and on which the excitation force of the field pole 3 acts, the main magnetic pole 4 overhanging the armature core 6; 7, an armature including the armature core 6; and 8, corner portions respectively formed in the overhanging sections of the main magnetic pole 4. Further, Lm represents the axial length of the main magnetic pole 4; Ln, the axial length of the straight section of the main magnetic pole 4; Lc, the axial length of the armature core 6; and R, the axial length of the corner (roundness) formed in the corner portion 8 or an axial chamfer dimension. Although the axial length Ln of the straight section of the main magnetic pole 4 is substantially equal to the length Lm of the magnet, and the magnet has a rounding at each corner portion 8 to prevent the magnet from chipping when it is assembled.
When the armature 7 supported on the inner periphery of the field pole 3 in the field device for the conventional magneto direct-current motor above is supplied with power, it is subject to the excitation force of the main magnetic pole 4 and rotates.
In this case, improvement in performance is to be made by justifying Lm&gt;Lc since the main magnetic pole 4 is a ferrite magnet.
Improvement in performance has thus been attempted by making the main magnetic pole 4 overhang the armature core 6 in such a conventional device. However, the magnetic flux in the neighborhood of the overhanging corner portions is not actually working effectively and these portions have been left as being useless; the problem is that these corner portions have accordingly resulted in not only increasing the weight of the device but also making it costly.